1. Field of the Invention
This invention relates to a measuring device comprising a measuring device body and a spindle held in the body to be movable in the axial direction for measuring a dimension of an object to be measured according to a displacement amount of the spindle in the axial direction. More specifically, this invention relates to a measuring device for detecting the displacement amount of the spindle in the axial direction as an electric signal by a detecting means including a sensor on stationary side attached to the measuring device body and a sensor on movable side provided on the spindle oppositely to the sensor on stationary side with a predetermined gap. This invention can be applied to a dial gauge, a linear displacement measuring device and the like.
2. Description of Related Art
Conventionally, a known displacement detecting measuring device includes a measuring device body, a spindle movably provided on the measuring device body and a detecting means for converting relative movement displacement amount between the spindle and the measuring device body into electric signal.
As above-mentioned detecting means, a conventional detecting means has a sensor on stationary side attached on the measuring device body and a sensor on movable side disposed on the spindle, both sensors being opposed at a predetermined gap therebetween, to convert a relative movement displacement amount into electric signal.
Specifically, above detecting means employs optical or electrostatic detecting method by using a sensor on stationary side (such as light casting and receiving portion or electrode) and a sensor on movable side (such as optical matrix and electrode) provided on the spindle at a predetermined gap against the sensor on stationary side.
And a digital display dial gauge which detects the displacement amount of the spindle movably disposed on the measuring body as an electric signal by aforementioned detecting means to transmit to an outside processor or to display digitally to a digital display provided on the measuring device body is known as a measuring device.
Such measuring devices are often used as in-line equipment (for simultaneous measuring on a process line) thus high durability is required. Accordingly, a stroke bearing (a bearing of which holding cylinder rotatably holds a plurality of balls thereon) is used as a bearing of the spindle to enhance durability.
In adopting the stroke bearing, a "double holding structure" in which two separate parts of the spindle with a detecting means disposed therebetween is held by the stroke bearings respectively and a "single holding structure" in which only one part of the spindle is held by the stroke bearing may be considered.
The "double holding structure" is, for instance, shown in U.S. Pat. No. 5,289,729. However, the axis line of the two stroke bearing is necessary to be aligned in the "double holding structure", resulting in intricate adjustment thereof. And the size of the device gets larger since the stroke bearings are disposed on the two parts of the spindle in the axial direction.
In the "single holding structure", the size of the device can be reduced since the stroke bearings are centralized to a single portion of the spindle even when a plurality of the stroke bearings are employed to be linearly connected. And Abbe's law concerning accuracy can be easy to attain since the detecting means can be disposed close to an axis line extending from the tip end of the spindle, with the reduction in size.
However, a rigidity at the tip end of the spindle is lowered in a type with long moving stroke of the spindle since the "single holding structure" is used, which result in lowering the rigidity in the detecting portion of the sensor on stationary side and the sensor on movable side.
Particularly, frequent vibration and impact are highly likely during the use in in-line equipment, and a gap between the sensor on stationary side and the sensor on movable side tends to fluctuate by the vibration and the impact. When the gap between the sensor on stationary side and the sensor on movable side fluctuates and goes beyond a predetermined range of the gap, the detecting means outputs error signal and miscount occurs in signal processor, thus failing to achieving stable and highly accurate measurement.
Accordingly, it is desirable to develop a measuring device with high anti-vibration and anti-impact property. For instance, in the U.S. Pat. No. 5,172,485, a spacer is provided to a gap between the sensor on stationary side and the sensor on movable side and each sensor is in close contact with each other having the spacer disposed therebetween, thereby always maintaining a gap corresponding to the spacer between both of the sensors.
However, a durability problem due to an abrasion of the spacer or the like occurs in above structure in which the spacers are always in close contact with each other.
The object of present invention is to solve above-mentioned conventional problems, that is, to provide a measuring device of which anti-vibration property and anti-impact property are improved, and at the same time, durability and reliability are possible to be enhanced.